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| United States Patent |
5,641,831
|
|
Hamilton
|
June 24, 1997
|
Anti-hazing silicone rubber article composition and process for making
same
Abstract
A silicone rubber article (such as a wiper blade) has a low process fluid
leach rate and high resiliency. The composition comprises, prior to cure,
approximately 100 parts by weight of a vulcanizable elastomer including at
least one silicone polymer, at least one crosslinkable process fluid, and
low pressure and high pressure peroxide catalysts to accommodate a
two-stage vulcanization process. According to the invention, the
composition provides anti-hazing qualities and confers hydrophobic
properties to a contacted surface. The polymer also includes about 50 to
220 parts by weight of a siliceous filler.
| Inventors:
|
Hamilton; James R. (Weatherford, TX)
|
| Assignee:
|
JMK International, Inc. (Fort Worth, TX)
|
| Appl. No.:
|
480539 |
| Filed:
|
June 7, 1995 |
| Current U.S. Class: |
524/588; 524/862; 528/15 |
| Intern'l Class: |
C08G 077/06; C08L 083/04 |
| Field of Search: |
524/588,862
528/15
|
References Cited
U.S. Patent Documents
| 3696068 | Oct., 1972 | Creamer | 525/477.
|
| 3884866 | May., 1975 | Jeram et al. | 524/703.
|
| 3972850 | Aug., 1976 | Hamilton et al. | 260/375.
|
| 4089833 | May., 1978 | Simpson | 578/32.
|
| 4339564 | Jul., 1982 | Okamura | 528/15.
|
| 4873274 | Oct., 1989 | Cummings et al. | 523/500.
|
| 4904434 | Feb., 1990 | Hyer | 264/146.
|
| 4981637 | Jan., 1991 | Hyer | 264/146.
|
| 5122562 | Jun., 1992 | Jeram et al. | 524/588.
|
| 5283927 | Feb., 1994 | Gibbon et al. | 15/250.
|
Primary Examiner: Lee; Helen
Attorney, Agent or Firm: Perkins; Jefferson
Parent Case Text
This is a continuation of application Ser. No. 08/194,791 filed on Feb. 10,
1994, which is a divisional of application Ser. No. 07/936,585 filed on
Aug. 26, 1992, both abandoned, the text of which is hereby incorporated by
reference.
Claims
What is claimed is:
1. A silicone rubber vehicle wiper blade, formed according to the following
process:
providing a compound having a major portion of a silicone polymer including
at least one polydiorganosiloxane having a viscosity of more than 500,000
centistokes, a minor portion of a hydroxy-ended crosslinkable silicone
process fluid having a viscosity of less than 100 centistokes, a low
pressure catalyst adaptable to promote silicone crosslinking at a first
predetermined temperature, a high pressure catalyst adaptable to promote
silicone crosslinking at a second predetermined temperature at least
110.degree. F. higher than said first predetermined temperature, and a
portion of filler including a small particulate filler having a size of
less than or equal to 5 microns;
elevating the compound to said first temperature;
responsive to elevating the compound to said first temperature,
crosslinking said silicone polymer and said process fluid using only said
low-pressure catalyst;
allowing the low pressure catalyst to decompose to inactive species
including gas-phase compounds;
allowing the gas-phase compounds to escape the compound at a sufficiently
slow rate that no blistering or bubbling occurs;
thereafter elevating the compound to said second temperature; and
responsive to elevating the compound to said second temperature, further
crosslinking said silicone polymer and said process fluid using said high
pressure catalyst.
2. The silicone rubber vehicle wiper blade of claim 1, wherein said high
molecular weight polydiorganosiloxane is present in the range of forty to
seventy percent by weight.
3. The silicone rubber vehicle wiper blade of claim 1, and further
including a major portion of siliceous or calcareous filler.
4. The silicone rubber vehicle wiper blade of claim 3, wherein said filler
is present in the range of twenty-five to sixty-five percent by weight.
5. The silicone rubber vehicle wiper blade of claim 1, wherein said
hydroxy-ended low molecular weight silicone process fluid is present in
the range of about three to ten parts by weight per hundred parts of said
silicone polymer prior to said steps of crosslinking.
6. The silicone rubber vehicle wiper blade of claim 1, wherein said
hydroxy-ended low molecular weight silicone process fluid comprises from
two to twenty percent hydroxy content by weight of the process fluid, and
from 0.2 to 13 percent mole vinyl content.
7. The silicone rubber vehicle wiper blade of claim 1, wherein the low
pressure catalyst is a diaroyl peroxide.
8. The silicone rubber vehicle wiper blade of claim 1, wherein the high
pressure catalyst is selected from the group consisting of dialkyl and
diaralkyl peroxides and mixtures thereof.
9. The silicone rubber vehicle wiper blade of claim 1, wherein said
polydiorganosiloxane has an average molecular weight on the order of
1,000,000, a viscosity prior to said steps of crosslinking of at least
500,000 centistokes, wherein the organo side groups of the
polydiorganosiloxane are from 0.02 to 1 mole percent vinyl and the rest
methyl, and wherein the polydiorganosiloxane is present in the gum in the
range of 40 to 70 percent by weight.
10. The silicone rubber vehicle wiper blade of claim 1, wherein said
hydroxy-ended silicone process fluid is a crosslinkable
polydiorganosiloxane having a hydroxy content of 2 to 20 percent by weight
of the process fluid, a vinyl content 0.2 to 13 mole percent with respect
to the total number of organo side groups on the last said
polydiorganosiloxane, an average molecular weight in the range of 1,000 to
10,000 and a viscosity prior to said steps of crosslinking in the range of
5 to 100 centistokes.
11. The silicone rubber vehicle wiper blade of claim 7, wherein said low
pressure catalyst is selected from the group consisting of
2,4-dichlorobenzoyl peroxide and dibenzoyl peroxide.
12. The silicone rubber vehicle wiper blade of claim 8, wherein said high
pressure catalyst is selected from the group consisting of
dimethyl-2,5-di-(t-butyl peroxy)hexane, di-t-butyl peroxide, dicumyl
peroxide, halogenated and organo substituted derivatives of dicumyl
peroxide, and mixtures thereof.
13. The silicone rubber vehicle wiper blade of claim 8, wherein the high
pressure catalyst is present in the compound prior to said step of
elevating the compound to said second temperature in the range of 0.5 to
1.5 parts by weight relative to 100 parts by weight of said silicone
polymer.
14. The silicone rubber vehicle wiper blade of claim 1, wherein the
low-pressure catalyst is selected from the group consisting of
2,4-dichlorobenzoylperoxide and dibenzoyl peroxide, the first temperature
being approximately 240.degree. F.
15. The silicone rubber vehicle wiper blade of claim 1, wherein the
high-pressure catalyst is selected from the group consisting of
dimethyl-2, 5-di-(t-butyl peroxy) hexane, di-t-butyl peroxide, dicumyl
peroxide, halogenated and organo substituted derivatives of dicumyl
peroxide, and mixtures thereof, the second temperature being about
340.degree. F.
16. The silicone rubber vehicle wiper blade of claim 1, wherein said small
particulate filler consists of 3 to 140 parts by weight of fume or
precipitated silica, relative to one hundred parts by weight of said
silicone polymer.
17. The silicone rubber vehicle wiper blade of claim 16, wherein said small
particulate filler includes from 3 to 55 parts of fume silica.
18. The silicone rubber vehicle wiper blade of claim 16, wherein said small
particulate filler includes no more than 36 parts of precipitated silica.
19. The silicone rubber vehicle wiper blade of claim 1, wherein said first
temperature falls in the range of 220.degree.-240.degree. F.
20. The silicone rubber vehicle wiper blade of claim 1, wherein said second
temperature falls in the range of 340.degree.-500.degree. F.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to silicone rubber articles and, more
particularly, to an improved silicone rubber article having anti-hazing
properties and a composition and process for making same.
BACKGROUND OF THE INVENTION
Silicone rubber, i.e., high molecular weight, vulcanized
polydiorganosiloxane, is able to withstand wide temperature variations
without an appreciable effect on its physical properties. Silicone rubber
is virtually unaffected by ultraviolet radiation, even over long periods
of time. It is also resistant to ozone, oil, salt, water and other road
and automotive chemicals. Silicone rubber has been used in windshield
wiper blades, gaskets, spark plug boots and weather stripping.
However, silicone rubber articles currently existing in the art tend to
leave an undesirable coating, or haze, on surfaces in which they come into
contact, such as windshields. The coating is caused by the leaching of a
low molecular weight silicone polymer process fluid, used to aid in the
manufacture of conventional silicone rubber articles. In the case of
silicone rubber wipers the process fluid leaches onto the windshield at an
unacceptably high rate, yielding a vision-obstructing haze. Experiments
indicate that a slower leaching process fluid would not create an
undesirable haze, but rather would confer beneficial hydrophobic qualities
and anti-hazing properties to the windshield surface.
Additionally, other than for sponge or foam applications, the prior art
teaches the use of a single peroxide, either a "high pressure" or a "low
pressure" peroxide, as the vulcanization catalyst for the high molecular
weight polydiorganosiloxane base. Low pressure peroxide catalysts include
diaroyl peroxides such as 2,4-dichlorobenzoyl peroxide. High pressure
catalysts include dialkyl and diaralkyl peroxides such as dicumyl
peroxide. Formulations using only one peroxide as a catalyst cause a less
than optimal amount of crosslinking, with less of a physical barrier to
the leaching of low molecular weight silicone process fluid. Wiper blades
cured with one catalyst therefore have less than optimal anti-hazing
properties.
Use of both a low-pressure and a high-pressure catalyst could theoretically
induce an increased degree of crosslinking or cure, and a concomitant
increase in elasticity, and resiliency, and decrease in tackiness and
compression set. One of the perceived problems in the prior art of using
both a "high pressure" and "low pressure" catalyst is that blistering or
bubbling by the low-pressure catalyst byproducts will occur at the high
temperature needed to "kick off" the high-pressure catalyst, such as
340.degree. F. While such a reaction is desirable for sponge or foam
applications, it is not desirable for articles requiring more structural
integrity, such as windshield wiper blades, spark plug boots, weather
stripping and gaskets.
A long felt need continues to exist for an improved silicone rubber article
that confers beneficial hydrophobic qualities and possesses slow-leaching
or anti-hazing properties. A need further exists for silicone rubber
articles having an increased degree of crosslinking or cure.
SUMMARY OF THE INVENTION
According to the invention, a silicone polymer composition is provided that
when formed into a wiper blade or other article and cured, confers
beneficial hydrophobic qualities, has slow-leaching or anti-hazing
properties and has enhanced resiliency and resistance to compression set.
According to one aspect of the invention, a silicone polymer composition is
provided that includes a base having at least one high viscosity, high
molecular weight polydiorganosiloxane with an average molecular weight on
the order of one million and a viscosity of at least 500,000 centistokes,
and a crosslinkable silicone process fluid, which has a molecular weight
and a viscosity substantially less than the polydiorganosiloxane. The
crosslinkable process fluid preferably has an average molecular weight in
the range of 1,000 to 10,000 grams per mole, a viscosity in the range of 5
to 100 centistokes, a hydroxy content of 2 to 20 percent by weight and a
mole vinyl content of 0.2 to 13 percent. The silicone polymer composition
includes at least one catalyst which is adaptable to crosslink both the
process fluid and the high molecular weight silicone polymer.
According to another aspect of the invention, a silicone rubber article is
provided which, prior to cure, contains a silicone polymer base with at
least one high molecular weight, high viscosity polydiorganosiloxane, a
"low pressure" catalyst, and a "high pressure" catalyst. The high pressure
catalyst may be a dialkyl or diaralkyl peroxide, and the low pressure may
be a diaroyl peroxide. These high pressure and low pressure catalysts
cause curing at different temperatures. The low pressure catalyst reacts
at a first, lower predetermined temperature such as 240.degree. F. and the
high pressure catalyst reacts at a second, higher, predetermined
temperature such as 340.degree. F. The curing process according to the
invention uses these two catalysts and occurs in two steps. In a first
curing step, the low-pressure/catalyst is activated at the first
predetermined temperature to effect at least a partial crosslinking or
cure of the base. In a successive second curing step, the high-pressure
catalyst is activated at the second predetermined temperature to effect
further crosslinking and complete the cure. Thus, the low-pressure and
high-pressure catalysts react with the base to produce a tightly knit
polymer which has improved resiliency and which leaches process fluid
slowly. As formed into a wiper blade, the cured silicone rubber imparts
desirable hydrophobic qualities to a contact surface.
In a preferred embodiment, the composition includes both a crosslinkable
process fluid, a low pressure catalyst and a high pressure catalyst. After
cure of the composition, a silicone rubber article is achieved with high
resiliency, and with at least a partially crosslinked process fluid
contained therein, such that the process fluid leaching rate is reduced
both by reduction in free process fluid and by further physical barriers
to leaching.
The present invention provides a composition that is useful in the
manufacture of windshield wiper blades, spark plug boots, weather
stripping and gaskets, and for other articles where exceptional resistance
to compression set, resilience and/or decreased process fluid leaching are
desirable. The present invention yields a commercially acceptable,
anti-hazing silicone wiper blade.
BRIEF DESCRIPTION OF THE DRAWINGS
Further aspects of the invention and their advantages may be discerned from
reading the following detailed description when taken in conjunction with
the drawings in which:
FIG. 1 is a perspective view of a silicone rubber vehicle wiper blade
according to the invention;
FIG. 2 is a transverse cross-sectional view taken substantially along line
2--2 of FIG. 1; and
FIGS. 3 and 4 are schematic block diagrams of a continuous extrusion
manufacturing processes employing the invention.
DETAILED DESCRIPTION OF THE INVENTION
Silicone rubber articles according to the invention are manufactured using
a silicone elastomer gum, which includes a crosslinkable process fluid and
at least one and preferably two peroxide catalysts. A filler of siliceous
and/or calcareous materials is also included.
Physical Structure
Referring to FIGS. 1-3 of the drawings, like numerals are used for like and
corresponding parts of the various drawings. FIG. 1 shows a perspective
view of a representative rubber article according to the invention, in the
illustrated embodiment a windshield wiper blade, shown generally at 10.
The body of wiper blade 10 includes a wiper superstructure retaining
element indicated generally at 11 and a squeegee blade 12. The
superstructure retaining element 11 includes a thin neck 14 and a
relatively thick or wide flange 16.
In a preferred embodiment, the squeegee blade 12 gradually decreases in
thickness between a thick base 18 and a relatively thin squeegee blade tip
20. Each side 22 of the squeegee blade 12 is inwardly arcuate from the
base 18 to the tip 20.
FIG. 2 shows a cross-sectional view of the wiper blade taken substantially
along line 2--2 of FIG. 1. As shown in FIG. 2, blade 10 includes a
superstructure-joining or retaining portion 11, a squeegee blade 12 and a
preferably flat wiping edge 20.
As shown in FIG. 2, the retainer 16 is defined by a restricted neck 14
formed by longitudinal grooves 23 at opposite sides of the neck 14. The
longitudinal grooves 23 extend the length of blade 10 on opposite sides
thereof thereby providing the opposite sides of the neck 14 with an
outwardly extending flange or retainer 16.
While the present invention is described in conjunction with a wiper blade
of a particular shape as illustrated in FIGS. 1 and 2, other silicone
rubber articles may be made according to the process of the invention and
using the disclosed compositions of the invention. For example, wiper
blades of the forms disclosed in application Ser. No. 07/708,245, assigned
to the assignee of this Application and filed on May 31, 1991, and U.S.
Pat. Nos. 4,981,637 and 4,904,434 issued to Hyer, commonly assigned to the
assignee of the present invention, may also be formed using the process of
the invention. The above-referenced application and issued patents are
fully incorporated herein by reference. The invention may also be used to
form other silicone rubber articles where high resiliency and/or low
process fluid leach rates are desirable, such as spark plug boots, engine
gaskets, other elastomer components for use in high temperature
environments, and weather stripping, to name a few such applications.
Composition
The composition according to the invention includes a silicone polymer base
or gum including one or more high molecular weight polydiorganosiloxanes,
a preferably crosslinkable silicone process fluid having a molecular
weight which is substantially less than the high molecular weight
polydiorganosiloxane and at least one catalyst adaptable to crosslink the
polysiloxanes of the base and process fluid. Preferably, the catalyst
includes a low pressure catalyst and a high pressure catalyst to
accommodate a two stage curing process. The gum is preferably cured first
at a low temperature and then at a high temperature.
One or more high molecular weight polydiorganosiloxanes may be used to make
up the silicone polymer base or gum. The organo side groups of the high
molecular weight polydiorganosiloxanes should have a very major portion,
such as at least 99%, of saturated side groups, and a very minor portion
of unsaturated side groups. For example, the side groups of the silicone
polymers can, when taken together, constitute between 99.00 and 99.98
weight percent methyl side groups. Taken as a whole, the high molecular
weight polydiorganosiloxanes have a vinyl, phenyl or other nonsaturated
side group percentage of 0.02 to 1.0 percent by weight of the total side
groups; a preferred range by weight of nonsaturated side groups is 0.1 to
0.3 percent of the total side groups.
While the base may have only one high molecular weight
polydiorganosiloxane, the base preferably includes a blend of two high
molecular weight silicone polymers: a major portion of
dimethylvinylsiloxy-ended polydiorganosiloxane, with the organo side
groups consisting of 99.8% weight methyl and 0.2% weight vinyl, and a
minor portion of dimethylvinyl-siloxy-ended polydimethylsiloxane. The
first component of this polymer blend, that is, the one with vinyl side
groups, may range in weight percent between 55 and 76 parts where both
constituents total 100 parts, while the second component may range in
weight percent between 24 and 45 parts. Each of these two constituents
typically have an average molecular weight of about one million, and
viscosities in excess of 500,000 centistokes. These high molecular-weight,
high-viscosity polydiorganosiloxanes are present in vulcanized form in the
cured article in the range of forty to seventy percent by weight.
The process fluid is a relatively low molecular weight, crosslinkable
polydiorganosiloxane with a portion of its organo side groups being
nonsaturated. The process fluid may be a hydroxy-ended
polymethylvinylsiloxane. The molecular weight of the process fluid is in
the range of 1,000 to 10,000 grams per mole and has a viscosity in the
range of 5 to 100 centistokes. Preferably the process fluid includes a
hydroxy content of 2 to 20% by weight and a mole vinyl content of 0.2 to
13%. The process fluid helps to prevent crumbling and premature hardening
of the uncured gum through hydrogen bonding. Generally, the higher the
percent hydroxy, the lower the quantity of process fluid required to
maintain a shelf life or processability window. Additionally, the higher
the vinyl percentage, the tighter the crosslink density and lower amount
of noncrosslinked process fluid, yielding a greater reduction in the low
molecular weight process fluid and other volatile loss (See Table V). In
silicone windshield wiper blades, it is the quick leaching (loss) of the
low molecular weight process fluid that causes the vision-obstructing
haze.
A preferred crosslinkable process fluid is a clear liquid, commercially
known as "Mobay Dispersing Agent SI205" (SI205). The physical properties
of SI205 include a specific gravity of 0.997; an SiOH content of 19% by
weight (.+-.1%); and an Si-vinyl mole content of 3.33% (.+-.0.3%). The
process fluid may be present in the gum in amounts ranging from 3 to 10
parts by weight relative to 100 parts by weight of the high molecular
weight polymers, and preferably is present at about 7 pph. The amount of
process fluid and the amount of reinforcing filler (discussed below) which
should be used are related, as the process fluid is used to coat the
siliceous or calcareous particles and prevent hardening from hydrogen
bonding. Ratios by weight of reinforcing filler to process fluid may vary
from three to one to about twelve to one, and preferably are around six to
one.
The peroxide catalyst may be chosen from the families of diaroyl, dialkyl
and diaralkyl peroxides, and mixtures of these. The diaroyl peroxide
catalysts are known as "low pressure" catalysts. Low pressure catalysts
include 2,4-dichlorobenzoyl peroxide, which is preferred, dibenzoyl
peroxide and mixtures thereof. One such low pressure catalyst is
commercially available as TS 50. The dialkyl and diaralkyl peroxides are
called "high pressure" catalysts. Usable dialkyl peroxides include
dimethyl-2,5-di-(t-butyl peroxy)-hexane, which is preferred, di-t-butyl
peroxide, or mixtures thereof. One such high pressure dialkyl peroxide is
commercially available as VAROX-P. Dicumyl peroxide is a preferred
peroxide of the diaralkyl type; but halogenated and organo substituted
derivatives thereof and mixtures of any of the foregoing could also be
used. One such high pressure diaralkyl peroxide is commercially available
as DICUP 40C. It is also possible to employ a mixture of dialkyl and
diaralkyl peroxide catalysts, such as a mixture of VAROX and DICUP.
The ratio of low to high pressure peroxide catalysts can vary from 15/85 to
80/20 by weight depending on the rheology, formulation and processing
temperatures preferred. The preferred ratio range is approximately 15/85
to 40/60. A mixture of both high and low pressure combinations could also
include the incorporation of platinum catalysts.
In order to optimize crosslink density and reduce volatile loss in a rubber
article such as a windshield wiper blade, a combination of high/low
pressure peroxides should be used. The high/low pressure cure (discussed
infra) in combination with a process fluid having a substantial amount of
vinyl side groups is the most efficient method of achieving a slow time
release of low molecular weight silicones.
The silicone bases according to the invention preferably include a
component of siliceous and/or calcareous or other filler. The filler
should be present in the gum in concentrations of between 40 and 220 parts
per hundred (pph) parts of the high viscosity polydiorganosiloxanes. Where
the cured article is a vehicle wiper blade, it is preferred that the
filler concentration should be in the range of 75 and 220 pph. Fillers for
this purpose may include from 25 to 200 pph of a large particulate filler
having an average diameter of 5 to 100 microns. Such a large particulate
filler can, for example, comprise any of several refractory oxides, such
as ground quartz, celite (diatomaceous earth), chalk and other siliceous
and calcareous minerals, ferrites, alumina, and mixtures thereof. The
particulate size of this large particulate component of the filler is
preferably in the range of 5 and 30 microns. Ground quartz is a
particularly preferred constituent for the large particulate filler
component. A second component of the filler has a much smaller size, on
the order of 5 microns or smaller. This filler component may comprise from
3 to 140 pph of a refractory oxide or mineral, such as fume or
precipitated silica. It is particularly preferred that from 3 to 55 pph of
fume silica be present in the small particulate component.
In addition to or in replacement of the fumed silica, precipitated silica
can be used. In preferred compositions, from 0 to 36 pph precipitated
silica may be used in the filler. As precipitated silica replaces fume
silica, relatively more precipitated silica should be used.
The relatively low molecular weight, crosslinkable hydroxy-ended
polydimethyl siloxane can be thought of not as a primary silicone
constituent of the silicone polymer gum, but instead as a process aid for
the coating of the fumed and precipitated silica. The crosslinkable
process fluid reacts with the silica surface to keep down hydrogen
bonding. It coats the filler surface. Otherwise, the added small
particulate filler makes the composition too hard. The high molecular
weight silicone polymers and the filler preferably account for at least 90
percent of the weight of the composition of the wiper blade or silicone
article. As a weight percentage of the cured article, the filler
components are present in the range of 25 to 65 percent.
Other components in a preferred composition include a stabilizing agent
that controls additional, unwanted vulcanization due to heat. Such a
stabilizer is cerium octoate, present in a concentration between 0.3 and
1.6 pph. The stabilizer does not interfere with the initial vulcanization,
but instead stabilizes the composition under warm or hot conditions. A
preferred concentration of cerium octoate is about 0.4 pph.
Finally, the composition may include a pigment, which can range from 0.2 to
20 pph. A preferred range of pigmentation is 0.6 to 2.7 pph. These
pigments should be peroxide-insensitive pigments and may comprise
inorganic oxides or, alternatively, certain organic compounds where
extremely bright colors are desired.
Process
According to one aspect of the invention, the silicone polymer is put into
a mold having the shape of the article desired and then passed to a curing
station.
In another embodiment, the single-stage curing process described above may
be replaced with a two-stage curing process. The first stage of the
two-stage curing process takes place at a first temperature below the
reaction temperature of the high-pressure catalyst, such as 220.degree. to
240.degree. F. After sufficient time for low-pressure cure, such as ten
minutes, the article is then passed to a second stage having a temperature
above the critical temperature of the high-temperature catalyst, such as
at or above 340.degree. F.
According to another aspect of the invention, silicone rubber articles may
be made by a continuous extrusion and cure process. Such processes are
fully disclosed in U.S. Pat. Nos. 4,981,637 and 4,904,434, both of which
are fully incorporated herein by reference.
FIG. 3 is a block diagram of such a continuous extrusion and one-stage
curing process as employing the invention. Where the article is an
automotive wiper blade or the like, a continuous length of compound
formulated according to the invention is extruded from an extruder 100.
The extruder may have a hopper (not shown) that feeds into a cold cylinder
(not shown). The cooled elastomer is then forced by one or more spiral
screws (not shown) out through a dye (not shown). The orifice of the dye
forms a cross-sectional shape of, for example, the wiper blade sought to
be manufactured (see, e.g., FIG. 2). Extrusion processes of the type
described are well known in the art and are discussed, for example, in
Lynch, W., Handbook of Silicone Rubber, and L. K. Arnold, Introduction to
Plastics, Iowa State University Press, (1968), pages 46-49. The extrusion
dye may be shaped to produce a pair of wiper blades in edge-to-edge
relation as is disclosed in either of U.S. Pat. Nos. 4,981,637 or
4,904,434.
The continuous length of extruded elastomer is passed to a curing station
102. This curing station can be a continuous vulcanizer employing a liquid
medium such as a eutectic salt bath through which the elastomer is drawn.
The salt bath is kept at a temperature of approximately 400.degree. to
500.degree. F., and preferably about 430.degree.. The velocity of the
continuous elastomer through the salt bath is controlled such that the
total cure time is approximately 1-2 minutes.
The continuous strip of the elastomer is next passed to a separator 106 in
the instance that the continuous strip of elastomer is formed in joined
pairs such as a pair of wiper blade strips. Various separating techniques
are known in the art; two of these are shown in U.S. Pat. Nos. 4,981,637
and 4,904,434. After the two wiper blade halves are longitudinally
separated from each other, the two strips of elastomer are passed to a
cutter 108, which cuts the wiper blade strips into appropriate sections
dimensioned to wipe automotive windshields. This completes the end of a
wiper blade squeegee manufacturing process.
FIG. 4 is a block diagram of an alternative extrusion and cure process.
This extrusion and cure process is generally similar to the one
illustrated in FIG. 3, with the exception that the curing process takes
place in two stages. The first stage takes place at a first curing station
102, while the second stage takes place at a second curing stage 104. The
first curing station may be a fluid which is kept at a temperature of
approximately 220.degree. to 240.degree. F., such that the first or
low-pressure catalyst is initiated. The velocity of the continuous
elastomer through the first fluid is controlled such that at least a
partial cure is obtained using the low-temperature or low-pressure
catalyst. After the first curing stage 102, the continuous strip of
elastomer is passed to a second curing station 104 which in general may be
of a structure similar to the first curing stage 102. The second curing
stage 104 may include a eutectic salt bath through which the elastomer is
drawn. The temperature of the second curing stage should be in the range
of 340.degree. to 500.degree. F. Curing stations 102 and 104 can also be
hot air vulcanizing tunnels. The curing stage 102 shown in FIG. 3 can
likewise be a hot air vulcanizing tunnel. After the second stage of cure,
the continuous stream of elastomer is treated as before.
The present invention solves the problem perceived with two-catalyst curing
processes. The prior art suggests that the low temperature catalyst would
create large volumes of gaseous byproducts as the article was subjected to
temperatures at or above 340.degree. F. The invention has demonstrated,
however, that the low pressure catalyst in the invention's curing process
causes no bubbles or blistering effects. Any substances formed as a
byproduct of the reaction of the low pressure catalyst form non-active
species which eventually boil off. The low temperature or low-pressure
catalyst is completely decomposed. The high temperature cure imparts
further resistance to deformation and compression set. It is thought that
the crosslinking created by the high-pressure catalyst may help to prevent
the blistering and bubbling of the gum caused by volatization of the
low-pressure catalyst by-products.
Further, the combination of the high and low temperature catalysts slows
the leaching of the silicone process fluid by at least one of two
mechanisms. First, since the crosslinking density produced by both the
high and low temperature catalysts is higher than that of the low
temperature catalyst alone, there is a greater physical barrier presented
to leaching process fluid. Second, since a crosslinkable process fluid is
employed, the low and high temperature catalysts act to crosslink the
process fluid either to other process fluid molecules or to the high
molecular weight polydiorganosiloxane matrix such that less free process
fluid is available to leach from the silicone rubber article.
Because of this combination of characteristics, wiper blades and other
silicone rubber articles can be manufactured having improved process fluid
leach rates. As used for the manufacture of wiper blades, for example, a
slower process fluid leach rate means that the free process fluid is
deposited upon an automotive windshield at a slower rate and for a longer
period of time. It is expected that such wiper blades will impart
hydrophobic properties to the automotive windshield glass which they
engage, yet will avoid the hazing problems associated with a high process
fluid leach rate experienced in certain conventional silicone wiper
blades.
EXAMPLES
Two groups of compounds were formulated using various combinations of
peroxide catalysts. The first group, consisting of samples labeled A-E,
included a base compound (J919) having a crosslinkable process fluid. The
second group, samples labeled F-J, included a base compound (J913) having
a noncrosslinkable process fluid.
The two base compounds, J919 and J913, used in the following tests are
disclosed chemically as follows. The term "pph" means parts per hundred by
weight.
______________________________________
J919 (pph)
J913 (pph)
______________________________________
Dimethyl methylvinyl polysiloxane
100.0 100.0
Organosilane ester 0.8 0.8
Hydroxyterminated dimethyl
0.0 7.0
polysiloxane (noncrosslinkable
process fluid)
Hydroxyterminated dimethyl
7.0 0.0
methylvinyl polysiloxane
(crosslinkable process fluid)
Precipitated silica
40.0 40.0
Cerium octoate in dimethyl
0.4 0.4
polysiloxane
Diatomaceous earth 0.0 2.0
Ethoxyterminated dimethyl
0.0 3.0
polysiloxane
______________________________________
As set forth in TABLE I below, samples A-E were formulated using 100 pph by
weight of the J919 base compound and listed quantities in pph of various
combinations of TS-50, VAROX P and DICUP 40C. The J919 compound includes a
crosslinkable process fluid, hydroxyterminated polymethylvinyl siloxane.
TS-50 is a commercially available low pressure peroxide catalyst. VAROX P
is a commercially available high pressure peroxide catalyst. DICUP 40C is
a commercially available version of dicumyl peroxide, which is another
high pressure peroxide catalyst.
TABLE I
______________________________________
A B C D E
______________________________________
J 919 100.0 100.0 100.0 100.0
100.0
TS-50 0.3 0.5 0.7 0.7 0.3
VAROX P 0.5 0.8 -- 1.0 1.5
DICUP 40C -- -- 0.5 -- --
______________________________________
Samples F-J, set forth in TABLE II, were formulated using 100 pph by weight
of the J913 base compound and various combinations of the low/high
pressure catalysts identified for TABLE I. The J913 compound includes a
noncrosslinkable process fluid, hydroxyterminated dimethyl polysiloxane.
TABLE II
______________________________________
F G H I J
______________________________________
J 913 100.0 100.0 100.0 100.0
100.0
TS-50 0.3 0.5 0.7 0.7 0.3
VAROX P 0.5 0.8 -- 1.0 1.5
DICUP 40 C -- -- 0.5 -- --
______________________________________
Example 1
The first experiment measured the hardness of each of the ten compositions,
samples A-J. One hundred grams of each of the ten formulas were
individually formed as sample "buttons". Each button was cured in a
circulating hot air oven for ten minutes at 240.degree. F., followed by
ten minutes at 340.degree. F. Following the two-stage cure, five
measurements were taken to determine the hardness of each sample. The
average durometer reading for each sample is recorded in Table III, below.
Sample E, which includes a crosslinkable fluid and a higher peroxide
content than the other samples, exhibited the greatest hardness. As
samples A-E illustrate, an increase in the hardness of the material,
indicative of crosslinking, is directly proportional to an increase in the
peroxide content. Samples F-J exhibited substantially lower durometer
readings, regardless of their respective peroxide content, due to the
absence of a crosslinkable process fluid.
TABLE III
______________________________________
Sample Durometer of Buttons
______________________________________
A 51
B 52
C 50
D 53
E 54
F 44
G 45
H 43
I 46.5
J 47
______________________________________
Example 2
Table IV, below, depicts the results of compression tests performed on the
above ten button samples, A-J. The original thickness of each button,
t.sub.o, was measured and recorded in the left column of Table IV. The
buttons were then placed in a circulating hot air oven having a
temperature of 350.degree. F. Each button having an original thickness of
t.sub.o +t.sub.s was compressed to approximately 50% of its original
thickness to a compressed thickness of t.sub.s. The compression was
relieved after a period of 22 hours. After compression, each button was
measured to determine its new thickness t.sub.i. The difference between
the original thickness, t.sub.o, and the new thickness, t.sub.i, indicates
the overall resiliency of each formulation. Greater resilience is
indicated as t.sub.i approaches t.sub.o. The resiliency, or memory, of the
material is indicative of its crosslinking.
The right-hand column of Table IV indicates the percentage of compression
that each button failed to recover, as represented by the following
formula:
##EQU1##
As illustrated, an increased peroxide catalyst content for those samples
including the crosslinkable process fluid, i.e., samples A-E, yielded as
increased resiliency of each button. In contrast, samples F-J, which did
not include a crosslinkable fluid, exhibited greater inelastic behavior.
Accordingly, better crosslinking yields greater resiliency, and higher
peroxide concentrations yield improved crosslinking capability.
TABLE IV
______________________________________
%
Compression
Sample t.sub.o
t.sub.s t.sub.i
Set
______________________________________
A .303 .141 .270 20.7
B .303 .141 .273 18.5
C .316 .141 .285 17.5
D .313 .141 .295 10.5
E .3155 .141 .296 10.8
F .3095 .141 .252 34.5
G .317 .141 .259 33.0
H .309 .141 .265 26.2
I .313 .141 .262 29.7
J .307 .141 .265 25.3
______________________________________
Example 3
The presence of vinyl within the crosslinkable process fluid acts with the
peroxide to prevent leaching of the process fluid. Table V, below,
illustrates the volatile loss over a two week time period. A small die was
used to cut out cylinders having uniform thicknesses and diameters so that
exposure to heat was as uniform as possible. These cylinders measured
approximately 0.1 inches in diameter with wall thicknesses of
approximately 0.075-0.088 inches. The cylinders were then placed in a
circulating hot air oven at 350.degree. F. The oven temperature was raised
to 400.degree. F. during the final two weeks of the experiment. The
specimens were weighed at various intervals over a 20 day period to
determine any mass loss. All weights are measured in grams with a margin
of error of .+-.0.01 g. The bottom row depicts total mass loss for each
sample.
Among samples A-E, total weight loss ranged from 0.0206 to 0.0304 grams.
Weight loss among samples F-J ranged from 0.0727 to 0.0836 grams. Notably,
the mass loss among the samples without the crosslinkable fluid, samples
F-J, was almost three times that of samples A-E, which included
crosslinkable fluid. Table V indicates that the presence of the vinyl
within the crosslinkable process fluid found in samples A-E crosslinks in
the presence of the peroxide catalyst such that leaching of the process
fluid is reduced. Due to such crosslinking, process fluid mass loss is
reduced.
TABLE V
__________________________________________________________________________
DATE A B C D E F G H I J
__________________________________________________________________________
TARE 1.5030
1.4782
1.5115
1.4555
1.4800
1.5033
1.3306
1.3245
1.3243
1.4882
day 1
2.8227
2.8235
3.1832
2.8630
3.0142
2.8950
2.8179
1.8477
2.8020
3.1002
day 3
2.8180
2.8166
3.1795
3.8597
3.0101
2.8844
2.8021
2.8307
2.8902
3.0920
day 7
2.8153
2.8143
3.1743
2.8574
3.0085
2.8775
2.7932
2.8225
2.8856
3.0852
day 8
2.8145
2.8129
3.1731
2.8557
3.0069
2.8722
2.7892
2.8171
2.8801
3.0811
day 12
2.8102
2.8059
3.1684
2.8489
3.0018
2.8593
2.7693
2.7913
2.8615
3.0628
day 14
2.8074
2.8026
3.1650
2.8443
2.9991
2.8511
2.7621
2.7829
2.8532
3.0552
day 20
2.8021
2.7931
3.1581
2.8349
2.9898
2.8166
2.7395
2.7541
2.8219
3.0275
Net Loss
.0206
.0304
.0251
.0281
.0244
.0781
.0784
.0836
.0801
.0727
__________________________________________________________________________
In summary, novel silicone polymer rubber compositions and articles have
been disclosed that when cured have increased resiliency, slower process
fluid leach rates and, as wiper blades, have anti-hazing properties. Novel
methods of manufacture are disclosed to create such articles.
While the present invention has been described with the aid of the above
detailed description and the examples, the invention is not limited
thereto but only by the scope and spirit of the appended claims.
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